Possibly alternator/regulator noise. If so, the noise is coming in on the power, not the ground or the audio inputs. You can buy/make a noise filter that goes on the input power to the equipment.
Might be a faulty audio cable(s) that is not adequately shielded.
Edit: Another possibility is that the speaker wires are acting as antennae. Keep them (and all other wiring) as short as possible.
Last edited by SouthArk370Z; 11-15-2019 at 09:01 AM.
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